MLVSS Calculator

Calculate Mixed Liquor Volatile Suspended Solids (MLVSS) in mg/L from the 550 °C ignition test or from MLSS and its volatile fraction.

Quick Facts

Method
Loss on ignition at 550 °C (Standard Methods 2540 E)
MLVSS = (mass burned off × 1000) ÷ sample volume in mL. Typical municipal ratio VSS/TSS ≈ 0.70–0.85.

Your Results

Calculated
MLVSS
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Volatile suspended solids (mg/L)
Volatile fraction
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MLVSS ÷ MLSS (VSS/TSS)

Ready

Enter your lab values and press Calculate.

What MLVSS is

MLVSS stands for Mixed Liquor Volatile Suspended Solids. In an activated-sludge wastewater plant, "mixed liquor" is the aerated mixture of wastewater and microorganisms in the aeration basin. The total suspended solids in that mixture are the MLSS (Mixed Liquor Suspended Solids); the volatile portion of those solids — the fraction that burns away when the dried sample is ignited at 550 °C — is the MLVSS. Because living cells and other organic matter combust at that temperature while inert mineral matter does not, MLVSS is used as a practical estimate of the active biomass carrying out biological treatment. It is reported in milligrams per litre (mg/L).

The formulas this calculator uses

The primary method is the gravimetric loss-on-ignition test defined in Standard Methods for the Examination of Water and Wastewater, procedure 2540 E. A measured volume of mixed liquor is filtered, dried at 103–105 °C, and weighed, then ignited to constant mass at 550 °C and reweighed. The mass burned off is the volatile solids:

MLVSS (mg/L) = ( (dish + dried solids) − (dish + ash) ) mg × 1000 ÷ sample volume (mL)

The factor of 1000 converts a per-millilitre mass in milligrams to a per-litre concentration (1 L = 1000 mL). The second method estimates MLVSS from an already-known MLSS and a volatile fraction:

MLVSS (mg/L) = MLSS (mg/L) × (VSS ÷ TSS)

Why operators track it

MLVSS is the biomass term in the two most important activated-sludge control numbers. The food-to-microorganism ratio, F/M = (flow × influent BOD) ÷ (aeration-tank volume × MLVSS), tells operators whether the bugs are being overfed or starved. Sludge age (mean cell residence time) is likewise based on the mass of volatile solids held in the system. Using MLVSS rather than total MLSS removes the inert mineral solids that do no treatment work, so the ratios reflect the living population more accurately.

Typical reference values

  • MLSS in a conventional aeration basin: roughly 1,500–4,000 mg/L; extended-aeration and MBR systems run higher.
  • MLVSS/MLSS (VSS/TSS) ratio: about 0.70–0.85 for a healthy municipal plant; industrial or high-inert wastewaters can be lower.
  • Ignition temperature: 550 ± 50 °C — the standard that distinguishes volatile from fixed (inorganic) solids.

Frequently Asked Questions

Is MLVSS the same as MLSS?
No. MLSS is the total mass of suspended solids in the mixed liquor, including inert mineral matter. MLVSS is only the volatile (organic) fraction of that mass — the part that burns off at 550 °C — and is used as a stand-in for the active biomass. MLVSS is always less than or equal to MLSS, and the ratio between them (VSS/TSS) is typically 0.70–0.85.
Why is the sample ignited at 550 °C specifically?
550 °C is the temperature specified in Standard Methods 2540 E because it reliably combusts organic (volatile) solids while leaving most inorganic (fixed) solids intact. Igniting much hotter can drive off carbonates and other mineral matter, overstating the volatile fraction; igniting cooler can leave organics unburned, understating it.
Can I just estimate MLVSS from MLSS?
Yes, as an approximation. If you know the MLSS and a representative VSS/TSS ratio for your plant (commonly 0.75–0.80), MLVSS ≈ MLSS × ratio. This is convenient for day-to-day process control, but the ratio drifts with influent character and inert loading, so periodic direct MLVSS lab tests are still needed to keep the assumed ratio honest.

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